Solar-receiver tubes are used in solar power plants, for example, parabolic-trough power plants, which convert solar energy into electric energy. In this type of a solar power plant, a parabolically curved reflector reflects the sunlight onto a solar-receiver tube positioned at its focal point. The main operational parts of a solar-receiver tube comprise a glass sleeve surrounding a metal absorber tube, by means of which solar energy is converted to thermal energy. The thermal energy is absorbed and stored by a fluid used as a heat transfer medium, which circulates in a closed pipeline system. By a heat exchanger the thermal energy is transferred from the fluid to water, which is then used to run a steam turbine producing electric energy.
The conversion process of solar energy to thermal energy by means of a solar-receiver tube is accompanied by a transmission loss concerning several elements of a glass sleeve which is a part of the solar-receiver tube.
Firstly, a part of the immitted light is reflected and absorbed by the wall of the glass sleeve. Therefore, at both surfaces of the glass sleeve, and anti-reflective coating is applied.
Secondly, a transmission loss is caused by a scratch-resistant coating, which is applied to protect the anti-reflective coatings at both surfaces. The increased durability of the anti-reflective coating is therefore accompanied by a decreased efficiency of the solar-receiver tube.
During the process of applying a coating to a glass sleeve in a coating tank forces take effect on the fragile glass sleeve, which can lead to its damage and breakage. Especially in areas, where interfaces designed to hold the glass sleeve in a predefined position are in close contact to the wall of the glass sleeve there is a risk of damage. In particular, this is the case during coating processes using dipping techniques when buoyancy forces the glass sleeve upwards and presses it against the contact interface.